1. Field of the Invention
The present invention relates to a radiation sensitive composition for color filters used for a color liquid crystal display device or a color image pickup tube element.
2. Description of the Background Art
A color liquid crystal display device is mounted on a portable personal computer. Because of an increased demand for color liquid crystal display devices in recent years, reduction in the cost of color liquid crystal displays has been strongly desired. In particular, color filters which are indispensable parts of the color liquid crystal display device are mainly manufactured by forming patterns utilizing photolithographic etching of a chromium thin film used as a black matrix. The manufacturing cost of this type of color filter is extremely high. In addition, the environmental problem caused by chromium waste fluid materials produced during manufacture of the color filter is a subject of public concern.
As a means for solving these problems, attention has been given in recent years to a method for using a radiation sensitive composition in which a black colorant is dispersed to manufacture the black matrix. In this method, a radiation sensitive composition in which a colorant with a black color, for example, is dispersed is first applied to a substrate, the resulting coated film is irradiated with light through a photomask to cure the exposed part, and the coating is developed using an alkali, for example, as the developing solution, to remove the part of the coating which has not been irradiated, thereby obtaining a specified black color pattern (black matrix). Then, a process comprising the application of a radiation sensitive composition in which a colorant with a red, green, or blue color is dispersed, light irradiation, and the development treatment is carried out for various colors to form pixel arrays of each color on the same substrate. A color filter is obtained in this manner.
However, when a black matrix is manufactured by using a black colorant, a significant amount of the colorant which is a non-photosensitive component must be used to secure a sufficient light shielding effect for the black matrix. A relative decrease in the amount of the photosensitive component and a resulting decrease in the sensitivity of the radiation sensitive composition is unavoidable. Furthermore, because the effective strength of radiation in coated films is reduced due to absorption of light by the colorant or the like, the radiation strength inevitably decreases toward the bottom of the coated film (i.e. the part close to the substrate surface). As a result, the reaction for curing the coated film tends to be insufficient close to the bottom. In some case, the bottom of the pattern may be hollowed out during development of the coating, thereby producing lacks, losses, and under-cuts in part of the formed pattern. In addition, when the effective strength of radiation is decreased, the rate of film retention after development and the strength of the pixels themselves may decrease. As a result, the pixels are easily damaged when a transparent conductive film or a liquid crystal aligning layer are formed over the color filter. Although these problems can be solved to some extent using a high energy radiation, this tends to decrease the production efficiency of the black matrix.
Accordingly, efforts have been made to develop a photopolymerization initiator with a high photo-sensitivity sufficient to provide a reaction for curing the radiation sensitive composition for color filters close to the bottom of the coating. One example is a composition comprising a binder polymer which contains a carboxyl group, a poly-functional acrylate compound such as pentaerythrithol hexacrylate, and a photopolymerization initiator. As the high sensitive photopolymerization initiator used for this type of radiation sensitive composition, 2,2'-bis(2-chlorophenyl)-4,4',5,5'-tetraphenyl biimidazole, 2,2'-bis(2,3-dichlorophenyl)-4,4',5,5'-tetraphenyl biimidazole, and the like, have been proposed (for example, Japanese Patent Applications Laid-open No 75372/1994, No.75373/1994, etc.). The use of 2,2'-bis(2-chlorophenyl)-4,4',5,5'-tetraphenyl biimidazole, 5,5'-tetra(alkoxyphenyl) biimidazole, 5,5'-tetra(dialkoxyphenyl) biimidazole, 2,2'-bis(2-chloro-phenyl)-4,4',2,2'-bis(2-chlorophenyl)-4,4',2,2'-bis(2-ch lorophenyl)-4,4',5,5'-tetra(trialkoxyphenyl) biimidazole, and the like as the photopolymerization initiator of the radiation sensitive composition used in a print or photoresist has also been proposed (e.g. Japanese Patent Publication No. 38403/1973 official report, Japanese Patent Application Laid-open No. 174204/1987).
However, these radiation sensitive compositions do not provide a sensitivity of a level sufficient to eliminate the use of the high energy radiation. The above-mentioned problems of lack or loss of patterns, low rate of film retention, and the decreased strength of pixels may still occur if the amount of irradiation is insufficient.
In particular, because the above-mentioned 2,2'-bis(2-chlorophenyl)-4,4',5,5'-tetraphenyl biimidazole is scarcely soluble in solvents, it is impossible to add a large amount of this compound to sufficiently increase the photographic sensitivity. Insoluble or precipitated compounds may cause production of foreign material which may produce projections on pixels which may impair the function of the transparent conductive layer of liquid crystal aligning layer coated on the color filter.
In addition, the radiation sensitive composition using the above-mentioned 2,2'-bis(2-chlorophenyl)-4,4',5,5'-tetraphenyl biimidazole, may leave substances which are insoluble in a developing solution in areas other than the area where the pixels are formed during the development process in the manufacture of color filters. The resulting color filters exhibit low light transmission on the whole. Use of such color filters in a color liquid crystal display device may result in decreased display contrast. In addition, pixels formed by a succeeding process on the part where insoluble material remains exhibit insufficient adhesion to the substrate due to the insoluble material. This may cause peeling of that part from the substrate and production of damaged parts in the color filters.
Development of a new photopolymerization initiator which can solve these various problems has therefore been desired. In spite of the extreme difficulty in anticipating the relationship between the structure of a photopolymerization initiator and its characteristics, development of a photopolymerization initiator which can provide the radiation sensitive composition exhibiting excellent sensitivity and contrast, which produces no insoluble material during the development treatment has been strongly desired.
The present invention has been achieved in view of this situation and has an object of providing a radiation sensitive composition for color filters which can exhibit excellent photographic sensitivity and contrast, but does not produce any insoluble material during the development treatment.